1. Field of the Invention
The present invention relates to a beam scan velocity modulation (SVM) apparatus, and more particularly to a beam scan velocity modulation apparatus with an SVM disabling circuit employed for picture sharpness enhancement.
2. Description of the Prior Art
It is well known that an improvement in apparent picture resolution can be achieved by the use of modulation of the beam scan velocity in accordance with the derivative of a video signal which controls the beam intensity. This video signal is known as the luminance signal and the derivative of the luminance signal is employed for the beam SVM. The beam SVM will improve the picture sharpness in a color television system employing a color kinescope.
Many modern color television receivers also employ alternate video sources. An example of such an alternate video source is commonly referred to as an on screen display (OSD) generator. The function of the OSD generator is to provide additional display informations to a viewer while viewing a typical television program. Thus, OSD generator provides for the display on the television screen of time, day, channel number and other various control informations.
In implementing OSD display, the OSD informations are presented as graphical data together with the normal pictures.
Techniques for generating this type of graphical data which is superimposed upon the television picture are well known in the art. Such techniques include OSD generators which count television scan lines and insert at the correct pixel locations the proper graphics to thereby display the time of day, channel number and words such as "CONTRAST", "COLOR", "MUTE" and so on. The use of an on screen display and an associated OSD generator requires the substitution of a different video signal or kinescope drive for the normal video signal which is being processed by the television receiver. In this manner, the pertinent information can be superimposed upon the viewed image.
A scan modulation circuit modulates the picture displayed on a display device in accordance with the video content of a first video signal. An alternate video signal possesses its picture informations displayed on the display device when the alternate signal is selected. The operation of the scan modulation circuit is varied in accordance with this selection. A problem may occur in regard to scan velocity modulation in television receivers which also include an alternate video signal source such as on screen display generator. As is known, the SVM apparatus operates to modulate the horizontal beam scan velocity in response to differentiated luminance information from the main video source. This modulation may occur prior to OSD deletion of the main luminance signal and insertion of the character signal.
U.S. Pat. No. 5,072,300 (issued to Mark R. Anderson) discloses a beam scan velocity modultion apparatus, which controls the current in a scan velocity modulation (SVM) coil by a blanking pulse in order to eliminate the effect of SVM artefact generation during the operation of an OSD generator in a television receiver. In this arrangement, a ghost image representative of the deleted portions of the main luminance signal may appear on the television screen near or behind the inserted OSD character information since the current flowing in the SVM coil is not completely controlled. The ghost image behind the characters generated by the OSD display appears as an outline of the picture contained in the deleted portions of the main luminance signal.